rear end ratio?

[71enthusiast]

I have 71 350 with a 3 speed. At 70 mph i am doing 3700 rpms which is rediculously loud. Id like some info on what gears i should run with if i were to change my current ones. I want to be able to pull something from time to time but i would still like something that isnt so loud. Any help would be great!

[oldblue1968chevy]

Id say you have 373 lol me too it sucks going fast!

Maybe swap to 308 (Would be from a auto 1/2 ton 350 maybe)? Someone else is more knowledgable than me on this

Or you can get a trans. with OD

[1968CHEVYC10]

My guess would be 4.56, if you've got 28" tires you'd be turning 3700rpm at 67mph, I don't know what size tires you have, but if they're slightly larger than 28" then that would get you your 70mph.

[matthew20061]

You could swap in a 4 Speed Auto or overdrive unit instead of messing with the gears that way you get good highway cruising RPM and good acceleration/towing.

[TwoFiftyShifter]

Sounds like 4.11 or 4.56.

[71enthusiast]

Alright just checked i have 27 inch tires with a saginaw 3 speed on the column. i wanna do something about my tranny but i dont want to cut my floor for a different transmission that has more speeds, is there an over drive kit i can get for my 3 speed or somethin?

[oldblue1968chevy]

There is a 'gear vendor od' whatever that is, not really sure..

I *THINK* there is a factory 3spd with OD but they are rare and pricey..

[cam.man67]

There was a factory 3-speed with an OD unit attached to it. I believe it was only available on trucks up till 1968, though Chevelles and I think maybe full-size vehicles were available with it. It only came on lesser engines, in the case of trucks, only with 6-bangers. i think.

[Blue Rat]

I have a 71 that came out with 3 speed on the column an a 307 with 12 bolt 4.11's in back.

Going to put an auto column an auto tranny in it in the next few months.

[Jacfourteen]

assuming your speedo is correct, it sounds like you have 4.11's. I would say if you want better cruise rpm and don't want to do OD then 3.08's would do it. Your off the line performance will suffer some though. Like stated above overdrive will give you better cruise, without the negative effects of higher gears. I have 4.56 in mine now and I can't stand cruising at higher than 60, I have a rear with 3.73's that I'm gonna swap in and after that I'm gonna do a OD tranny.

[cparman]

go to this site:
http://www.hurst-drivelines.com/
and look for their RPM CALCULATOR
Plug in the things that you know, and it will give you an idea of what you have. then plug in a 3.08 gear and you can see the difference. It might help you to be sure that you will get to where you want to go.

[Longhorn Man]

what size truck do you have? (C/10, 20, 30?)
What kind of tach you have?

[treveiger]

Different online rpm calculators end up different sometimes. The boardmember raycow gave me this to figure it out.

I can't understand why different online calculators should give you different answers, but I always do mine with a hand-held calculator.

The magic number is 20168. Divide your tire diameter into that to get tire revolutions per mile, and then multiply the result by your axle ratio. The number you get will be driveshaft revs per mile, which is the same as engine revs/mi if you don't have overdrive or torque comverter slip.

As an example, let's say you have 31" tires and a 4.10 rear end, so:

20168/31 = 650.6 tire revs/mi
650.6 x 4.10 = 2667

2667 engine revs/mi. That is also your engine rpm at 60 mph. Other speeds are proportional, so if you want the engine speed at 70 mph:

2667 x 70/60 = 3111 rpm at 70 mph.

Id say you have 4.11s or 4.56, most likely 4.56. If you switched to 3.08s youd be running 2684rpms at 70, maybe look at getting bigger rear tires when the time comes to change them out to. To switch rear end gears from what im hearing its best to switched just the whole rear end out. I hope this helps

[71enthusiast]

I have a c10 with a 5000 rpm tach. Ill do the math when I get back home but I can sure smoke my tires haha I appreciate the help!
Posted via Mobile Device

[71enthusiast]

Alright heres my math-
20168/27 inch tires=746.96 tire revs/mi
746.96x4.10 gears=3062.54 engine revs/mi at 60
3062.54x1.17(70/60 rounded)=3,572.96 engine revs at 70
however my truck does 3700 rpms at 70 granted that it is right..so lets work backwards-
3700rpms@70 /1.17(70/60 rounded)=3162.39 rpms @ 60
3162.39/746.96 tire revs/mi = 4.23 gears
4.23 gears does that sound right?

[tqlspec]

you probably have 4.11 rearend. When you calculate wheel diameter you need to use the actual rolling diameter. Mark the ground where the tire meets the ground and roll the truck forward or backward until the tire has made one revolution. Mark the ground and measure the distance between the 2 marks. It'll give you the true wheel diameter.

[71enthusiast]

cant i just go 3.14 x diameter to get the circumfrence of the wheel
3.14 x 27=84.78

[Longhorn Man]

I would get a better tach than a stock style one and recheck, or just pull the cover off the diff and find out exactly what you have in there.
I'm willing to bet a kick in the daddy parts you have something a little more normal like a 3.73 and your RPMs are more like 2900-3100

[eagleuh1]

Check to see if theirs a tag on the cover first. Might still be there.

[Mike C]

Is this a 3/4 ton truck? 2 wheel drive? The Dana axle would have had a tag between two of the bolts saying what the gear ratio was (or at least how many teeth on the ring and how many on the pinion), the Eaton does not in my experience.

Was this an original V8 truck? Any optional gear ratio would be on the SPID. Most likely a 6 cylinder 3 speed 3/4 ton would have had 4.57, but at 40 years old, who knows what has been swapped.

The easiest way to lower your highway cruising speed is to put some taller tires on it. A 30"-31" tire will make a big difference, but not as much as you need. The next easiest would be a rear axle swap to something with much lower gears. 3.54 Dana 60 if a 3/4 ton or 3.07-3.42 if a 1/2 ton.

[Jacfourteen]

Originally Posted by 71enthusiast

Alright heres my math-
20168/27 inch tires=746.96 tire revs/mi
746.96x4.10 gears=3062.54 engine revs/mi at 60
3062.54x1.17(70/60 rounded)=3,572.96 engine revs at 70
however my truck does 3700 rpms at 70 granted that it is right..so lets work backwards-
3700rpms@70 /1.17(70/60 rounded)=3162.39 rpms @ 60
3162.39/746.96 tire revs/mi = 4.23 gears
4.23 gears does that sound right?

That's the same answer I got when I figured it my own way, I figured 4.11 was closest and not too far off considering margin of error of tach and speedo. I say if you have never cracked the case on the rear, you might as well because its not that hard and it's good to change out the fluid and it takes out all the guessing of what ratio is in there.